Refinishing my basement here in PA. Divided the basement into halves and finishing one half with other unfinished (storage, work bench, etc). Removed 18yold tank water heater from the new "finished" area and replaced house hot water with a tankless now on the unfinished side. With the new gas Navien tankless I put in a water softner in front of it from the cold house supply. They are now both located on the "unfinished" side for aesthetics.

The problem I am trying to solve is: I am getting a low flow - drop in pressure in the house fixtures when I get a second shower turned on, a flushed toilet, sink faucet turned on... or other fixtures... etc. Not an increase in hot temp with a cold fixture being turned on. Just a whole house drop in pressure.. hot and cold... to the point where being in the shower with hardly any pressure stinks.

BTW the problem was present before I put the tankless in and still had the old 40gal tank HW running.. but not necessarily there before I started "reconfiguring" the pipe layout of the house. My finishing of the basement has been in stages... so not all done at the same time. The reconfiguring is so I can have my cold water come in and now travel half way across the house to the "unfished" area with the softner and then out to the HW heater and also supply "soft" cold water to all the house fixtures. The whole house is 3/4 inch supply with just 1/2 inch tapping off close to supply the respective house fixtures.

Some background troubleshooting:
Ive done some trouble shooting, and with the softner bypassed... still the same problem. When I had the plumber come in to reroute the gas supply to the tankless, I asked him about the problem. He suggested that I had a failed pressure reducing valve after the water coming into the basement (cold house supply). I know we had "hard" water for many years before the softner, so I could see that this could be a problem. Basically mineral scaled up and clogged the PRV screen. I recently bought a new PRV and put it in. (I didnt feel like messing with trying to fix the old one internally... so replacing was easier for me). It didnt fix the low pressure problem.

Now there are 3 things I have hunches on that could be problematic.. but this is where I could use some expertise advice.

1) When reconfiguring the cold layout to travel to the unfinished side of the basement... The old layout had cold coming in and "T"ing off into 2 directions. Roughly half of the house fixtures on one side of the "T" the other half on the other side of the "T". One shower on one side and the other on the other half of the split. I still effectively have this configuration... but now it is on the far side of the softner, rather than before it... before remodeling. However... again with the softner bypassed... no difference with the low pressure problem.

2) When I did the reroute.. I used 3/4 PEX rather than copper. So at some point the 3/4 copper gets transitioned to 3/4 PEX and then traverses to the softner. Output of the softner is 3/4 PEX also... then I transitioned back to the 3/4 copper that was already there originally. So I noticed (after the fact) that the copper to PEX transition and any ball shutoff valves I put in for the 3/4 PEX line use brass connections which fit inside the 3/4 PEX tubing for crimping... these fittings are about the same diameter as a 1/2 copper pipe.... give or take. So could this be a problem? 3/4 copper supply narrowing down to effectively 1/2 brass that fits inside the 3/4 PEX pipe. This happening a couple of times in the water chain. But no 1/2 pipe in this flow... just notice that the transition fittings are not truly 3/4 diameter. BTW... the licensed plumber I had in for the gas hookup .. didn't think this was a problem.

3) Could this be a external house problem? I noticed in the summer... the neighbors sprinkler hardly drops when watering the garden... where ours... drops when someone uses a fixture in the house. If so... how do I address this? Call the water company? How would they troubleshoot this? Of course I will most likely get charged.

Sorry for so long of a description, but I thought I should give a thorough background of the problem and some troubleshooting I did before I just asked the question of "How do I fix a low pressure/flow" problem?".

I could conceive this as the problem... but wouldn't this be an inherent problem with PEX in general? Wouldn't others have this same problem with using 3/4 PEX and any type of inline shutoff valve and/or a coupler in general? I would think this would be a huge problem in most PEX applications.

If someone was using 1/2 PEX for a run and used a shutoff valve before the fixture.... the inline valve would reduce it smaller than 1/2 internal diameter.... possibly down to 3/8ths (a guess)?

I guess all along I felt like this could be the culprit. I figured in a last resort I would go back and replace the PEX with copper to see if this fixes the problem. I just thought that it seems like a bad design for PEX. When you design a 3/4 inch "pipe" it would and could replace any existing 3/4" copper pipe. Couplings and all.

I guess I keep thinking of the case where if you had a 3/4" PEX line coming in off a main line into the house (say after the house meter) and feeding the house as a main water source. Whatever downline it goes to or stays at.... 3/4" or 1/2" house branches, manifolds, etc... If I were to take this main 3/4" PEX feed and want to put a whole house 3/4" PEX ball valve shutoff in this line/feed.... it would have the same low flow problem for the house. The valve would have the smaller 1/2"ish ends to it and restrict flow.

Or is this hypothetical situation... just poor design knowing the details of PEX tubing?

BTW.. Thanks for your replies... I'm not arguing.. just trying to justify the limits here of PEX.